In providing signals to and receiving signals from systems and integrated circuits, techniques have been developed to increase the signaling or data rate. In one known approach, serial data is transmitted over a high frequency serial differential signal interface. Differential signaling transmits a pair of normally opposite polarity signals that are separated by a relatively small differential voltage. The differential signal receiver senses the differential voltage and can recover clock and data signals from these received inputs. Transmitters can similarly send differential signals corresponding to data signals on interface. Because the differential voltage transmitted on this interface requires less than a full logic level voltage signal swing to transition from a high to a low signal level, higher speed switching may be achieved. These high frequency signals may be transmitted at data rates of up to 10 Gigabits per second (Gps). As technology advances, these data rates are continuously increasing. Further, by using groups of these multi-gigabit differential signals, very high data transmission rates may be achieved.
FIG. 1A depicts a voltage waveform of typical signals on a prior art multi-gigabit signaling interface. In FIG. 1A, the two opposing polarity signals are designated TX+ and TX−. The differential voltage, which may be for example around 2 Volts, represents signal data. When the differential voltage between the two signals is positive, it can represent one data value, for example, a logical “1”. When the differential voltage between the two voltages is negative, the signal can represent another data value, for example, a logical “0”. These logical value assignments are arbitrary and may be reversed, as is known in the art.
In certain signaling standards that utilize multi-Gigabit transceivers (MGTs), out-of-band (OOB) signals are used to communicate messages that are of types other than for transmitting and receiving high speed data. For MGT interfaces, an OOB signal is a signal which is sent with both of the normally opposite polarity differential signal lines at the same voltage. As non-limiting examples, OOB signals may be used to initialize a device at power up or reset, set certain parameters in the MGT buffers or otherwise set or read other parameters in the device, perform testing or configuration operations, and the like.
During OOB signaling, the two normally opposing differential signal lines are placed at the same potential, that is, the differential voltage is held below a low voltage threshold and approaches zero. The absence of a difference in these two voltages makes detection of the presence of an OOB signal fairly straightforward and robust.
FIG. 1B depicts an OOB signal on a multi-gigabit signal interface. As shown in the figure, when the two normally opposite polarity signals TX+ and TX− take approximately the same voltage value, that is when the differential voltage is less than a threshold voltage, the presence of an OOB signal can be detected. The OOB signals are typically transmitted at relatively low frequency and may provide, for example, initialization, observation, test and reset type functionality to the system. By using the OOB signals, additional input, output or input/output capability is provided to the device or circuit without the need for providing or using any additional pins or terminals.
Presently, new differential signaling standards based on MGTs with OOB signaling are being developed. As a non-limiting example, the bus standard known as Intel QuickPath interface presently in development includes a MGT interface using an OOB signaling protocol. The details of this OOB protocol are not yet known, so existing MGT transceiver buffers with OOB circuitry cannot support signaling using these as yet undefined OOB protocol signals. In addition other OOB signaling protocols that are defined in the future cannot be received by the prior art MGT transceivers. This characteristic of the prior art MGT transceivers necessitates a circuit redesign to receive these developing OOB protocols.
A continuing need thus exists for transceiver circuitry and methods to provide circuitry for receiving, transmitting and/or transceiving differential signals on a multi-gigabit signal interface that is further capable of receiving, transmitting or transceiving OOB signals on the multi-gigabit signal interface. The OOB signals may include OOB signal protocols that are not yet defined.